Heretofore, as a process for producing a polymer particle, a mechanical pulverization (or crushing) method has been utilized, which comprises, for example, crushing a polymer or polymer composition coarsely by using a crushing machine or other means, then pulverizing the crushed matter finely by using a jet mill or other means, and classifying the resultant by an air classifier or other means. In such a method, however, manufacturing machines are expensive, and additionally, thus obtained particle is irregular in shape and widely varies in particle size. In order to make the polymer particle size uniform, the obtained particle should be classified. However, unusable polymer particles in size are produced in large quantities by classification, and are unfavorable from an economical viewpoint. Further, spherical fine particles are preferred from the viewpoint of blocking among particles, dispersibility, flowability or others, however, it is impossible to obtain spherical particles by mechanical pulverization (or crushing) methods.
Moreover, Japanese Patent Application Laid-Open No. 176065/1998 (JP-10-176065A, Patent Document 1) discloses a process for obtaining a spherical fine particle of a thermoplastic resin (a), which comprises melt-kneading the thermoplastic resin (a) to be powdered with other one or more of thermoplastic resin(s) (b) to give a polymer composition comprising the resin (a) constituting the dispersed phase and the resin (b) constituting the continuous phase, and washing the polymer composition with a solvent incapable of dissolving the resin (a) and capable of dissolving the resin (b). In this process, however, it is necessary not only that the dispersed phase and the continuous phase are immiscible with each other, but also that an appropriate combination of the resin constituting the continuous phase with the solvent is selected depending on the kind of the resin of the dispersed phase. Therefore, the combination of the resins should be limited to a specific one, and in addition, the combination of the resin with the solvent should be limited to a specific one. Moreover, in the case where a fine particle comprising a plurality of resins is produced by using this process, not only the combination of resins constituting the dispersed phase and the combination of the resin constituting the dispersed phase and the resin constituting the continuous phase, but also the kind of the washing solvent or others are highly limited. In particular, in such a system, it is very difficult to obtain a multiple particle having a core-shell structure. Furthermore, the resin constituting the continuous phase is to be recovered in the end, or to be discarded in a dissolved state, as it is not included in the resin fine particle as a final product. However, recovery of the resin in the solution not only is very difficult but also is a caused factor of increase in the production cost of the polymer particle. Moreover, in the case of discarding the resin solution directly as a waste fluid, adverse effects on the environment cannot be ignored.
Japanese Patent Application Laid-Open No. 13816/1985 (JP-60-13816A, Patent Document 2) proposes a process for producing a thermoplastic polymer particle, which comprises melting a polyethylene glycol and a thermoplastic resin with stirring, putting the molten mixture into water to solidify, and then removing the polyethylene glycol from the resulting matter with water. Japanese Patent Application Laid-Open No. 9433/1986 (JP-61-9433A, Patent Document 3) discloses a process for producing a thermoplastic polymer particle, which comprises melting a thermoplastic resin and a polyethylene oxide with stirring, then cooling the molten mixture, and removing the polyethylene oxide from the mixture with water. Japanese Patent Application Laid-Open No. 165457/1997 (JP-9-165457A, Patent Document 4) discloses a process for producing a polymer fine particle, which comprises mixing a melt-formable and water-soluble polymer (such as a polyvinyl alcohol-series polymer, a denatured starch, or a polyethylene oxide) and a thermoplastic resin to give a melt-shaped product, and then removing the water-soluble polymer from the shaped product with water.
Even in these processes, however, since it is necessary that a resin and a water-soluble polymer are immiscible with each other, a selectable combination of the resins is limited, and in addition, the particle size distribution of thus resulting polymer particle is insufficient in uniformity. In particular, in the case of forming a polymer particle comprising a plurality of resins, the species of raw material to be used is highly limited because the combination of the resin and the water-soluble polymer also exerts a great influence on formation of the polymer particle. Further, these water-soluble polymers having small solubilities to water need a large amount of water for dissolution, and in addition, significantly deteriorate productivity of polymer particles due to the low velocity of dissolution. Furthermore, since such water-soluble polymers are often derived from unnatural products, a waste fluid containing such a water-soluble polymer dissolved therein adversely affects on the environment.
On the other hand, as a process for obtaining a polymer fine particle having a core-shell structure, a seeded emulsion polymerization, comprising polymerization of a polymerizable monomer after synthesizing a polymer fine particle as a core, is most popularly practiced. For example, Japanese Patent Application Laid-Open No. 70255/1995 (JP-7-70255A, Patent Document 5) discloses a production process of a core-shell polymer having an alkyl acrylate-series rubber-like core and a methyl methacrylate-series glass-like shell.
In the seeded emulsion polymerization, however, it is difficult to stably produce a particle having a particle size over 1 μm and a narrow particle size distribution, further the polymer available for the seed polymerization is limited to a polymer obtained from a radical-polymerizable monomer. Therefore, the seeded emulsion polymerization cannot be employed to a polymer obtained by a condensation reaction, or a curing or crosslinking reaction.
[Patent Document 1] JP-10-176065A (Claim 1)
[Patent Document 2] JP-60-13816A
[Patent Document 3] JP-61-9433A
[Patent Document 4] JP-9-165457A
[Patent Document 5] JP-7-70255A (Claim 1)